This invention relates to a Longitudinal Field Driven Field Asymmetric Ion Mobility (FAIM) filter, and more particularly to a micromachined spectrometer.
The ability to detect and identify explosives, drugs, chemical and biological agents as well as air quality has become increasingly more critical given increasing terrorist and military activities and environmental concerns. Previous detection of such agents was accomplished with conventional mass spectrometers, time of flight ion mobility spectrometers and conventionally machined FAIM spectrometers.
Mass spectrometers are very sensitive, highly selective and provide a fast response time. Mass spectrometers, however, are large and require significant amounts of power to operate. They also require a powerful vacuum pump to maintain a high vacuum in order to isolate the ions from neutral molecules and permit detection of the selected ions, and are also very expensive.
Another spectrometric technique which is less complex is time of flight ion mobility spectrometry which is the method currently implemented in most portable chemical weapons and explosives detectors. The detection is based not solely on mass, but on charge and cross-section of the molecule as well. However, because of these different characteristics, molecular species identification is not as conclusive and accurate as the mass spectrometer. Time of flight ion mobility spectrometers typically have unacceptable resolution and sensitivity limitations when attempting to reduce their size, that is a drift tube length less than 2 inches. In time of flight ion mobility, the resolution is proportional to the length of the drift tube. The longer the tube the better the resolution, provided the drift tube is also wide enough to prevent all ions from being lost to the side walls due to diffusion. Thus, fundamentally, miniaturization of time of flight ion mobility systems leads to a degradation in system performance. While conventional time of flight devices are relatively inexpensive and reliable, they suffer from several limitations. First, the sample volume through the detector is small, so to increase spectrometer sensitivity either the detector electronics must have extremely high sensitivity, requiring expensive electronics, or a concentrator is required, adding to system complexity. In addition, a gate and gating electronics are usually needed to control the injection of ions into the drift tube.
FAIM spectrometry was developed in the former Soviet Union in the 1980""s. FAIM spectrometry allows a selected ion to pass through a filter while blocking the passage of undesirable ions. One prior FAIM spectrometer was large and expensive, e.g., the entire device was nearly a cubic foot in size and cost over $25,000. These systems are not suitable for use in applications requiring small detectors. They are also relatively slow, taking as much as one minute to produce a complete spectrum of the sample gas, are difficult to manufacture and are not mass producible.
Moreover, the pumps required to draw a sample medium into the spectrometer and to provide a carrier gas can be rather large and consume large amounts of power. And, the carrier gas necessarily must flow in the same direction as the ions which requires a structure which separates the analytical gap from the ionization source.
It is therefore an object of this invention to provide a FAIM filter and detection system which can more quickly and accurately control the flow of selected ions to produce a sample spectrum than conventional FAIM devices.
It is a further object of this invention to provide such a filter and detection system which can detect multiple pre-selected ions without having to sweep the bias voltage.
It is a further object of this invention to provide such a filter and detection system which can even detect selected ions without a bias voltage.
It is a further object of this invention to provide such a filter and detection system which can detect ions spatially based on the ions"" trajectories.
It is a further object of this invention to provide such a filter and detection system which has a very high resolution.
It is a further object of this invention to provide such a filter and detection system which can detect selected ions faster than conventional detection devices.
It is a further object of this invention to provide such a filter and detection system which has a sensitivity of parts per billion to parts per trillion.
It is a further object of this invention to provide such a filter and detection system which may be packaged in a single chip.
It is a further object of this invention to provide such a filter and detection system which is cost effective to implement and produce.
It is a further object of this invention to provide such a filter and detection system which does not require the high flow rate, high power consumption pumps normally associated with spectrometers.
This invention results from the realization that the pumps used to draw a sample media such as a gas into a FAIM spectrometer and to provide a flow of carrier gas can be made smaller or even eliminated in part by the incorporation of an ion flow generator which creates a longitudinal electric field in the direction of the intended ion travel path to propel the ions to the detector and through the transversely directed asymmetric electric field which acts as an ion filter.
The result is the ability to incorporate lower cost, lower flow rate, and smaller, even micromachined pumps; a decrease in power usage; the ability to now apply clean filtered gas (e.g., dehumidified air) in a direction opposite the direction of ion travel to eliminate ion clustering and the sensitivity of the spectrometer to humidity. Separate flow paths for the source gas and the clean filtered gas are no longer required thus reducing the structure required to maintain separate flow paths taught by the prior art. Moreover, if an electrospray nozzle is used as the ionization source, the electrodes used to create the fine droplets of solvent can be eliminated because the electrodes which create the longitudinal and transverse electric fields function to both transport the ions and to create the fine spray of solvent droplets.
The spectrometer can be made extremely small, if required, and used in chemical and military applications, as a filter for a mass spectrometer, as a detector for a gas chromatograph, as a front end to a time of flight ion mobility spectrometer for increased resolution or as a filter for a flexural plate wave device.
The invention results from the further realization that an extremely small, accurate and fast FAIM filter and detection system can be achieved by defining a flow path between a sample inlet and an outlet using a pair of spaced substrates and disposing an ion filter within the flow path, the filter including a pair of spaced electrodes, one electrode associated with each substrate and a controller for selectively applying a bias voltage and an asymmetric periodic voltage across the electrodes to control the path of ions through the filter.
The invention results from the further realization that by providing an array of filters, each filter associated with a different bias voltage, the filter may be used to detect multiple selected ions without sweeping the bias voltage.
The invention results from the further realization that by varying the duty cycle of the periodic voltage, no bias voltage is required.
The invention results from the further realization that by segmenting the detector, ion detection may be achieved with greater accuracy and resolution by detecting ions spatially according to the ions"" trajectories as the ions exit the filter.
This invention features an ion mobility spectrometer comprising an ionization source for ionizing a sample media and creating ions; an analytical gap; an ion filter disposed in the analytical gap downstream from the ionization source for creating an asymmetric electric field to filter the ions; an ion flow generator for creating an-electric field in a direction transverse to the asymmetric electric field which is in the longitudinal direction for propelling ions through the asymmetric electric field; and an ion detector for sensing ions not filtered by the ion filter.
The ion detector is typically located proximate to the ion flow generator. The spectrometer may be a radiation source, an ultraviolet lamp, a corona discharge device, or an electrospray nozzle.
The ion filter is preferably connected to an electric controller for applying a bias voltage and an asymmetric periodic voltage to the ion filter. The ion filter typically includes a pair of spaced electrodes for creating an asymmetric electric field between them. The ion flow generator typically includes a plurality of spaced discrete electrodes insulated from these electrodes for creating the transverse direction electric field which propels the ions through the asymmetric electric field and to the detector.
Alternatively, the ion flow generator includes spaced resistive layers and a voltage is applied along each layer to create the longitudinally directed electric field which propels the ions through the asymmetric electric field and to the detector.
In another embodiment, the ion filter includes a first plurality of discrete electrodes electrically connected to an electric controller which applies an asymmetric periodic voltage to them. The ion flow generator includes a second plurality of discrete electrodes dispersed among the electrodes of the ion filter and connected to a voltage source which applies a potential gradient along the second plurality of discrete electrodes.
The analytical gap typically is enclosed by a housing. The ion filter includes electrodes on an inside surface of the housing and the ion flow generator includes electrodes proximate but insulated with respect to the ion filter electrodes. The ion detector also includes electrodes on an inside surface of the housing proximate to the ion filter and the ion flow generator.
The analytical gap is typically enclosed by a housing, the ion filter may include electrodes on an outside surface of the housing and the ion flow generator then includes resistive layers on an inside surface of the housing. A voltage is applied along each resistive layer to create a longitudinal electric field. Alternatively, the ion filter and the ion flow generator are combined and include a series of discrete conductive elements each excited by a voltage source at a different phase.